Monitoring equipment for security protection systems, of the type having a fixed voltage power source supplying current via a communications line to protected premises where the current may be varied to indicate a security breach, commonly utilizes electrical relays for automatic internal switching upon variation of the line current. These relays, though not easily damaged by high currents induced by lightning, have an inherent time delay and are not transparent to the remainder of the system.
Monitoring equipment using electronic solid state circuitry has been used, but has been subject to component damage from high currents induced by lightning and false reactions due to noise and transients. These may be eliminated by introduction of a resistor-capacitor filter which filters out short duration current signals, as in U.S. Pat. No. 3,252,156 to Muehter and U.S. Pat. No. 3,786,501 to Marnerakis, but any such filter reduces the security effectiveness of the equipment.
Regulatory groups, such as Underwriters Laboratories, Inc., require that security protection monitoring apparatus operate accurately even though its power supply voltage may vary widely. U.S. Pat. No. 3,786,501 to Marnerakis, which utilized operational amplifier input circuitry, does not recognize this problem, nor does it disclose a solution.